Communications devices can be used to transmit outgoing signals to another communications device and/or to receive incoming signals from another communications device. For example, digital subscriber line (DSL) devices (e.g., a DSL modem) can be used to transmit outgoing DSL signals to other communications devices and/or to receive incoming DSL signals over electrical wires such as copper telephone lines. At increased cost and improved efficiency, conventional direct current (DC) step-down Buck regulators used in DSL devices typically operate at a frequency that is within the DSL data modulation frequencies. However, operating a power supply of a DSL device at these switching frequencies can reduce the quiet line noise (QLN) performance levels.
To reduce interference with the operation of a communications device, the switching frequency of a switched mode power supply of the communications device is set to be higher than the data modulation frequencies of the communications device. For example, to reduce interference with the operation of a DSL device, the switching frequency of a switched mode power supply of the DSL device can be set to be higher than the DSL operating band. However, a power supply circuit, such as a Buck regulator, will typically require a larger voltage head-room to operate at the higher switching frequency. When a power supply circuit is switched to a battery back-up, the voltage head-room is more limited compared to a power outlet that can supply an unlimited amount of power. For example, for a residential gateway that includes a battery backed-up, the minimum voltage head-room may not be adequately maintained as the battery is depleted.